The present invention relates generally to optical systems and, more particularly, to an apparatus for combining the outputs from multiple gain elements of an array within a single resonator cavity.
Semiconductor lasers utilizing GaAlAs, GaAs, InGaAs, and other materials have been developed that have found application in a variety of fields including telecommunications, medical testing/treatment, optical measurements, and optical disk reading/writing. Such lasers can be designed with either a single gain element or with an array of discrete gain elements. In order to utilize such lasers, it is often desirable to couple the output from several discrete gain elements into a single output beam, thereby achieving greater output power, spectral bandwidth, beam brightness, etc.
U.S. Pat. No. 5,802,092 discloses a diode laser source in which the output from each of a plurality of concurrently driven laser gain elements within a single diode array are passed through suitable beam filling and focusing optics in order to converge the outputs to a single overlapping spot. Each of the laser gain elements is individually addressable, thus allowing individual elements to fail with only a marginal effect on the optical power and brightness of the overlapping spot. As a result of this design, the lifetime of the device is improved.
U.S. Pat. No. 5,513,201 discloses a system designed to achieve an increased energy density at the light focus of a linear semiconductor laser array. The system utilizes an optical path rotating device interposed between a pair of collimating elements. Each of the collimating elements is designed to provide collimation along a single axis. The system also includes a focussing element for condensing the laser beams that have been collimated in both directions.
U.S. Pat. No. 4,494,235 discloses a frequency stabilized semiconductor laser which utilizes a Fourier diffraction grating with a corrugation having continuous first order differential coefficients. In operation, the semiconductor laser emits a beam from one facet into the diffraction grating. The diffracted light from the grating is fed back into the semiconductor laser in such a way as to cause the semiconductor to emit an output beam with a stable wavelength from another facet. The grating can be rotated to achieve different wavelengths.
U.S. Pat. No. 5,276,695 discloses a tunable, optically pumped, solid state laser which simultaneously emits light at one or more wavelengths. The resonator uses two or more end reflective elements, two curved fold mirrors, and an output coupler reflective element. A wavelength dispersive element, e.g., a prism, is disposed in the reflective path in the laser resonator cavity between one fold mirror and the end reflective element, the wavelength dispersive element providing wavelength tuning capability. In order to optimize the wavelength dispersion performance of the dispersive element, the laser cavity mode is collimated as it passes through the element.
U.S. Pat. No. 5,541,946 discloses a laser which simultaneously emits light at two or more wavelengths. The laser includes at least two gain elements pumped by a single optical pumping source, each gain element generating a different wavelength. The resonator uses two plane highly reflective end elements, three concave fold elements, and an output coupler. A wavelength dispersive element, e.g., a prism, is disposed in the reflective path in the laser resonator cavity to provide a spatially separate path for each of the simultaneously emitted wavelengths. In order to optimize the wavelength dispersion performance of the dispersive element, the laser cavity mode is collimated as it passes through the element.
A tunable semiconductor laser with a single output is disclosed by Soole et al. in Wavelength-selectable Laser Emission from a Multistripe Array Grating Integrated Cavity Laser, Applied Physics Letters 61 (23), Dec. 7, 1992. In the disclosed system, an array of active laser elements is optically coupled to a fixed, etched-in, diffraction grating. Wavelength tuning is accomplished through selective activation of individual laser elements, the wavelength being determined by the position of the activated laser element relative to the etched-in grating.
Although a variety of intracavity beam combiners have been designed, these devices typically require very precise alignment. Accordingly, what is needed in the art is a robust intracavity beam combining system utilizing multiple discrete gain elements and a resonator structure that is primarily contained within a single optical element, thus minimizing system alignment requirements. The present invention provides such a device.
The present invention provides an apparatus for combining the outputs from multiple gain elements of an array within a single resonator cavity using a diffraction grating. The apparatus can be used to achieve a predetermined spectral output or to achieve a relatively high output power from an array of low power laser elements. The multiple gain element array can be a semiconductor diode laser array, an array of side or end pumped solid state laser materials, or a fiber laser array.
In at least one embodiment of the invention, the resonator cavity is comprised of a reflector, preferably deposited on the back facets of the gain element array, and an output coupler. Interposed between the gain element array and the output coupler is an approximately xc2xc pitch GRIN lens and a reflective diffraction grating, the reflective diffraction grating coupled to the back surface of the GRIN lens. The output coupler is preferably coupled to an optical fiber.
In at least one other embodiment of the invention, interposed between the gain element array and the output coupler of the resonator cavity is an optical element in which the back surface is shaped and onto which a highly reflective coating is deposited. Coupled to the front surface of the optical element is a diffraction grating. An entrance aperture on the front surface of the optical element allows light from the gain element array to enter the optic. The light from each element of the array is reflected off of the back surface of the optic onto the diffraction grating, reflected by the diffraction grating back towards the back surface of the optic, and focused by the back surface of the optic onto the output coupler.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.